The present invention relates to deep u.v. photosensitizers which are polycyclic compounds containing a cyclopentane-2-diazo-1,3-dione structural unit and to photosensitive compositions incorporating these compounds.
Photoresists are materials which change their solubility in a developer solution after it has been exposed to actinic radiation, such as ultraviolet radiation. Photoresist compositions comprise a photosensitive compound, a film forming polymeric resin and a solvent. The photoresist composition is applied to a substrate which is to be patterned and the solvent is then removed, leaving the photoresist as a thin film covering the substrate. As a consequence of the exposure to radiation, a different solubility rate results between imagewise exposed and unexposed portions of a resist film that yields a surface relief pattern after the development. Those photoresists which become more soluble in a developer solution in the exposed regions are referred to as "positive" photoresists Those which become less soluble in the exposed regions are referred to as "negative" photoresists.
Positive photoresists typically comprise an aqueous alkali soluble resin, such as novolak resin or polyvinyl phenol and a diazonaphthoquinone sulfonic acid ester sensitizer. The resin and sensitizer are applied such as by spin-coating, spray coating, or other suitable means from an organic solvent or solvent mixture onto a substrate, such as a silicon wafer. After imagewise exposure, the developer removes the non-image areas of the coated photoresist to produce a relief pattern in the photoresist film.
It is essential that the mask pattern be accurately reproduced in the substrate etch pattern. To achieve this high degree of accuracy, the mask pattern must be well resolved by the photoresist layer. The laws of optics and diffraction dictate that resolution will improve as the wavelength of the irradiation is shortened. Thus, photoresists capable of operating in the deep ultraviolet region (200-300nm) will be capable of higher potential resolution than those resists limited to operating in the near ultraviolet region (300-400nm).
Diazonaphthoquinone sulfonic acid esters are commonly used as sensitizers in conventional near u.v. photoresists. While these ester sensitizers are photoactive in the deep u.v., they exhibit several serious limitations for use as photosensitizers. These sensitizers exhibit intense absorptions in the deep u.v. region, making the resist composition excessively absorptive as well. These deep u.v. absorptions are also poorly photobleached by the exposing radiation so that the film's absorbance is not greatly diminished during the irradiation process. Ideally, the sensitizer photoproduct should be nonabsorbing in the region of irradiation used to expose the resist so that all absorbed light does useful chemistry, thereby maximizing sensitivity. These prior art sensitizers also possess near u.v. absorption bands which allow them to be used in conventional near u.v. photoresists. However, this near u.v. response would be considered a drawback in a true deep u.v. resist, as it would necessitate filtering the exposure source to remove long wavelength radiation to prevent degradation of the resolution.
This invention provides photosensitive compounds and formulations which contain sensitizers designed to operate effectively in the deep u.v. spectral region during the process of integrated circuit manufacture.
Other attempts have been made to design a photoresist system for the deep u.v. region. For example, UK patent Application No. 2,099,168 teaches ortho-nitrobenzyl groups attached to polymers and sensitizer molecules. The use of orthonitrobenzyl chemistry for photoresists is also taught in U.S. Pat. No. 3,849,137.
Chemistry suitable for deep u.v photoresists also includes that of chain scission of high molecular weight polymer into lower molecular weight polymer. In this case, the energy of the deep u.v. light is sufficient to rupture bonds in the polymer chain, resulting in lower molecular weight material of increased solubility. The most common example of this technique uses poly(methyl methacrylate). The primary drawback of this resist is the need for high exposure doses to yield a sufficient difference in molecular weights and the need to use an organic solvent as the developing medium.
Another example of deep u.v. photoresist technology involves the use of deep u.v. excimer lasers of high instantaneous fluence to ablate away selected areas of the photoresist film. This laser photoablation is disclosed in U.S. Pat. No. 4,414,059. Still another disclosure relating to deep u.v. sensitizers is found in European Patent Application 0 129 694, which describes compounds based on diazohomotetramic acid for use in photoresist compositions. U.S. Pat. No. 4,339,522 discloses 5-diazo meldrumacids; EP84 105 544 discloses diazohomotetramic acid compounds. U.S. Pat. No. 4,735,885 discloses deep u.v. photoresists formed from 1,3 disubstituted-5-diazobarbituric acid. Compounds having the 2-diazo-1,3,-dione grouping are also known, as shown in U.S. Pat. No. 4,622,283; 4,284,706; 4,339,522; 4,624,908 and Japanese Patent 78127439. Each of the foregoing has its own shortcoming such as volatility, stability and solubility.
The present invention provides improved compounds based on 5-membered ring structures. They have peak absorptions in the deep u.v. region and are especially sensitive to 248 nm eximer laser radiation.